From Halopedia, the Halo wiki
The Clockwork Rat is, as his name implies, a mechanical rat built from various mundane objects. His ears are made of open safety pins; his tail is made from a rubber wiper-blade; his whiskers are made from wires; his eyes are made from ball-bearings; his claws are made from surgical scissors; his teeth are made from the scraps of tin from under Perdita's father's workbench. Additionally, the rat's voice is a high-pitched metallic squeak. While without an actual sex the rat is often referred to with male pronouns.
The Clockwork Rat is a deceptive and manipulative character, a representation of Dr. Catherine Halsey and the SPARTAN Program. When caught by Perdita, he promises to help her survive and eventually find her way to her family. However, the rat secretly destroys the trail of red balloons she leaves, and in time alters her into a hideous cyborg while she sleeps.
List of appearances